4 Wire RTD Transmitters

The two red wires are common, and the two white wires are common. There should be next-to-nothing ohms across the two whites, and across the two reds, and the value of the ohms across any white and any red should be related to the temperature, as measured on a pocket multimeter.

The purpose of the multiple wires is simple. The RTD is wired into the instrument in a Wheatstone Bridge-type arrangement. Having the extra wires simply allows the effects of the wire resistance to be nullified, improving the correlation between the actual temperature and the indicated temperature.

Thanks for your help but MY question is "how do I connect my decade box up to these wires?"....Thanks J1.

How did you do it with a 3-wire RTD?

From the dec box..the single red onto No3 & the parallel pair off black onto 1&2..J1.

Not 100% sure what you're on about, but sounds like you should treat the whites as you did the blacks, and just take one (either) of the reds to No3.

If I join the 2 reds together and leave them on terminal 3 and connect my dec box onto 1 2&4 is that the same as a 3 wire cal???. J1.

Not quite - just use one red.

Edit - you should get the same result whichever one you use, but if you use both, the lead resistance in that leg will be very slightly lower.

I'm lost, the tx has 2 white wires and 2 red wires. My dec box has 3 wires of which 2 are in parallel, NEXT...please explain. J1.

Thanks!!!.....

Hi One & All,
Four Wire RTDThe 4 wire RTD uses only 1 current source. A current is injected through

WR1, the RTD and WR2. WR4 and WR3 is connected to a very high impedance

differential amplifier. Because there is no current flowing through WR3 and WR 4, there

is no voltage drop across them and the amplifier inputs sees only the voltage directly

across the RTD.This method is not affected by the difference in wire and connection

resistances.Therefore this is a good choice for sensors with non-equal lead lengths or

with connections resistances that could change over time.

BEN

The RTD is a duel RTD. It reads from one red and one white. the other red and white is a spare when the 1st one fails during operation. Thus you do not have to remove the RTD and replace it as you have a spare. Usually this is used in babbit bearings and you do not have to remove the bearing, just switch wires ect:

Hi One & All,

I spent many years on ships work with 4 wire RTD's and Never had one that was Dual RTDs but that not saying there is not one out there! The unit were always in side shell (Connection Head) with the RTD down a thermowell. The top of the shell had a terminal board to connect the unit to the wiring system and the RTD went down the small pipe and touch the sealed end of the pipe (Thermowell). If the RTD went bad, it could be removed with out opening the system that it was measuring the the air.

http://www.rtdcompany.com/items/index.cfm?CAT_ID=15

RTD hookup can be via 2, 3 or 4 wires to the J5 connector. The meter applies an excitation current of 196 μA (Pt 100 and Ni 120) or 5 mA (Cu 10). In 4-wire hookup, different pairs of leads are used to apply the excitation current and sense the voltage drop across the RTD, so that the IR drop across the excitation leads is not a factor. http://www.laurels.com/pt100.htm?src=adcenter

Four wire RTDs provide slightly better compensation, but are generally found only in laboratory equipment and other areas where high accuracy is required. When used in conjunction with a 3-wire instrument, a 4 wire RTD will not provide any better accuracy. If the fourth wire is not connected, the device is only as good as the 3 wire RTD; if the fourth wire is connected, new errors will be introduced. Connecting a 3 wire RTD to a 4 wire RTD instrument can cause serious errors or simply not work at all, depending on the instrument circuitry. A 2 wire RTD can be used with either a 3-4 wire instrument by jumping the appropriate terminals, although this defeats the purpose and reintroduces the uncompensated resistance of the leads. To get the optimum performance, it is generally best to specify the RTD according to the instrument manufacturer's recommendations

http://thermometricscorp.com/rtdsensors.html

The BLACK wire can be WHITE!

4-wire RTD circuits not only cancel lead wires but remove the effects of mismatched resistances such as contact points. A common version is the constant current circuit shown here. Is drives a precise measuring current through L1 and L4; L2 and L3 measure the voltage drop across the RTD element. Eo must have high impedance to prevent current flow in the potential leads. 4-wire circuits may be usable over a longer distance than 3-wire, but you should consider using a transmitter in electrically noisy environments

Is - is current source

Eo - is voltage indicating the temperature

http://www.tempco.com/Sensors/rtd_circuits.htm

I hope this helpful and clears things up. Again, I have work on ship sensor of over 10 years but I still come across new thing every day or something I have never hear or seen before.

BEN

Hi, Ben - glad you came back on that - your previous post, rambling on about WR1...WR4 made not a lot of sense (unless all these "WR"s were defined) - I was tempted to slap an OT on it.

Have a GA for this (more sensible) response.